In recent years, design work using a parametric 3D-CAD (three-dimensional computer-aided design) is mainstream in the field of mechanical design. In this parametric 3D-CAD, individual parts are managed using independent files, and an assembly (assembled parts) constituted of individual parts is also managed using a file different from these files. The assembly holds parameters indicating constraint conditions, such as distance, angle, and coincidence, called “assembly constraint”. The relative position relationship among the parts is determined using such parameters. These constraint conditions can reflect screw fastening, latch engagement, etc. of actually manufactured products. Therefore, the use of the constraint conditions allows positioning according to actual-good design intention. In a 3D model to which constraints are properly given, the constraints act in accordance with the geometry of the parts to automatically update the relative position among the parts to the design intention.